The present invention relates to a proton conducting polymer, a method for producing the same, a solid polymer electrolyte and an electrode.
In recent years, fuel cells have occupied an important position as next generation type clean energy sources. Above all, a solid polymer electrolyte type fuel cell is one in which both anode and cathode electrodes are each arranged across a solid polymer electrolyte membrane intervening therebetween. For example, in the case of a direct methanol type fuel cell (hereinafter referred to as a xe2x80x9cDMFCxe2x80x9d) in which methanol is used as a fuel, methanol is supplied to the anode side, and oxygen or air to the cathode side, thereby allowing electrochemical reaction to occur to generate electricity. Solid polymer electrolyte membranes having high proton conductivity have been developed for retaining the characteristics of their high output and high energy density, and for obtaining small-sized, lightweight fuel cells. The solid polymer electrolyte membrane used in the DMFC is required to have the barrier property to fuel methanol, that is to say, reduced permeability (cross-over) of fuel methanol from the anode side of the membrane to the cathode side thereof.
Previously, hydrated membranes of perfluorosulfonic acid polymers such as Nafion (trade name) manufactured by E. I. du Pont de Nemours and Company) have generally been used as the solid polymer electrolyte membranes. The structure of Nafion is represented by the following general formula (I): 
The above-mentioned hydrated membranes of perfluorosulfonic acid polymers have high proton conductivity, and the proton conductivity is exhibited by the generation of a channel structure caused by hydration (conduction of hydrated protons). That is to say, the conduction of protons takes place through water as a medium in the hydrated membranes of perfluorosulfonic acid polymers, and a specified amount of water exists in the hydrated membranes. Accordingly, methanol having high affinity with water easily passes through the membranes, so that the hydrated membranes of perfluorosulfonic acid polymers have a limitation with regard to the methanol barrier property.
Besides, as a polymer having proton conductivity independent of water, polybenzimidazole (PBI) doped with a strong acid such as phosphoric acid (hereinafter referred to as xe2x80x9cacid-doped PBIxe2x80x9d) is known. Specifically, PBI includes poly-[2,2xe2x80x2-(m-phenylene)-5,5xe2x80x2-bibenzimidazole] represented by the following general formula (II): 
With respect to the conduction mechanism of the above-mentioned acid-doped PBI membrane, it is said that proton hopping occurs through acids coordinated to Nxe2x80x94H groups contained in PBI, a base polymer, and that the proton hopping does not accompany movement of water. Then, the acid-doped PBI membranes have been expected to be significantly low in the cross-over amount of methanol, and excellent in methanol barrier property. However, the acid-doped PBI membranes have the disadvantage that elimination of dopants such as inorganic acids is liable to occur in an atmosphere of water/methanol (liquid fuel).
The present inventors have previously invented acid-doped PBI membranes in which dopant elimination is difficult to occur, and which are excellent in methanol barrier property by using diphenylphosphoric acid as a dopant in an amount of one molecule per Nxe2x80x94H group in PBI (Japanese Unexamined Patent Publication No. 2000-38472).
For improving the proton conductivity of the above-mentioned acid-doped PBI membranes, it is preferred that the Nxe2x80x94H group density of the base polymers is increased and that the density of acid components coordinated to the Nxe2x80x94H groups is increased. Further, for conducting protons in the solid polymer electrolyte membranes, the base polymers preferably have a low glass transition temperature (Tg) and a flexible molecular structure. Furthermore, from the viewpoint of chemical stability required for the solid polymer electrolyte membranes used in fuel cells, the proton conducting polymers are preferably aromatic polymers.
PBI that has hitherto been used as the acid-doped PBI membranes has imidazole rings, and two nitrogen atoms having unshared electron pairs exist in each imidazole ring. One nitrogen atom exists as an Nxe2x80x94H group, and the other nitrogen atom constitutes a double bond. The unshared electron pair of the nitrogen atom constituting the double bond contribute to the formation of a xcfx80 electron resonance structure of the imidazole ring. However, the unshared electron pair of the nitrogen atom of the Nxe2x80x94H group of the imidazole ring is kept free. It is therefore presumed that the substantial electron arrangement of the nitrogen atom of the Nxe2x80x94H group of the imidazole ring approximates to the electron arrangement of a nitrogen atom of an Nxe2x80x94H group connecting two aromatic rings.
A polymer having such an Nxe2x80x94H group connecting two aromatic rings is a polyaniline. The molecular structure thereof is simpler than that of PBI, and the Nxe2x80x94H group density thereof is high. The polyanilines include a polyaniline in which aromatic rings are bonded at the para-positions (hereinafter referred to as a xe2x80x9cpara type polyanilinexe2x80x9d), and a polyaniline in which aromatic rings are bonded at the meta-positions (hereinafter referred to as a xe2x80x9cmeta type polyanilinexe2x80x9d). The para type polyaniline has a xcfx80 conjugate structure, so that itself has electrical conductivity. Accordingly, although an acid-doped para type polyaniline obtained by doping the para type polyaniline with an acid component shows proton conductivity, it can not be used as a material for the solid polymer electrolyte membrane used in the fuel cell.
On the other hand, the meta type polyaniline can not have a xcfx80 conjugate structure, so that it can not exhibit electrical conductivity as it is. Further, synthesis thereof is difficult, and therefore only a few examples of electrolytic polymerization of aniline under special conditions are reported for synthesis methods thereof [T. Ohsaka et al., J. Electroanal. Chem., 161, 399 (1984), A. Volkov et al., J. Electroanal. Chem., 115, 279 (1980), and Onuki, Matsuda and Koyama, Nippon Kagaku Kaishi, 11, 1801 (1984)].
However, the meta type polyaniline has proton selective permeability (proton conductivity). Accordingly, there is an example in which it is evaluated as a PH sensor usable in metal ion-containing solutions [Onuki, Matsuda and Koyama, Nippon Kagaku Kaishi, 11, 1801 (1984)].
The meta type polyaniline has a flexible molecular structure, compared with the above-mentioned para type polyaniline. From the above, the meta type polyaniline having no electrical conductivity and having the flexible molecular structure is anticipated to exhibit the proton conductivity by acid doping, and to be applied as a novel solid polymer electrolyte material for fuel cells.
Furthermore, as an electrode used in a solid polymer electrolyte type fuel cell, a so-called MEA (membrane electrode assembly) is known. In the MEA, electrodes are formed of fine noble metal catalyst particles supported on carbon, a solid polymer electrolyte component formed on surfaces of the fine catalyst particles, and a fluorine resin for adhering the fine catalyst particles to one another. The electrodes are each arranged on two main planes of a solid polymer electrolyte membrane, thereby constituting a fuel cell (Japanese Unexamined Patent Publication No. 5-36418).
It is also conceivable that the above-mentioned acid-doped polyaniline (hereinafter referred to as an xe2x80x9cacid-doped polyanilinexe2x80x9d), that is to say, the proton conducting polymer, is used as the solid polymer electrolyte component formed on the surfaces of the fine catalyst particles, when it is high in proton conductivity.
The present invention has been made against a background of the current problems of the proton conducting polymers as the solid polymer electrolyte materials as described above, and attention has been given to the meta type polyaniline having the molecular structure desirable for the proton conducting polymers.
An object of the invention is to provide a proton conducting polymer.
Another object of the invention is to provide a method for producing the same.
A further object of the invention is to provide a solid polymer electrolyte comprising the proton conducting polymer, which is excellent in proton conductivity, methanol barrier property and stability of dopant in an aqueous solution of methanol.
A still further object of the invention is to provide an electrode comprising the proton conducting polymer and fine catalyst particles carried on porous particles.
The present inventors have conducted intensive investigation for attaining the above-mentioned objects. As a result, the inventors have discovered that a proton conducting polymer, a method for producing the same, a solid polymer electrolyte comprising the proton conducting polymer, which is excellent in proton conductivity, methanol barrier property and stability of dopant in an aqueous solution of methanol, and an electrode comprising the proton conducting polymer and fine catalyst particles carried on porous particles are provided by doping a meta type polyaniline with an inorganic acid or an organic phosphoric acid compound, thus completing the invention.
The invention provides a proton conducting polymer comprising a polyaniline.
It is preferred that 70 mole percent or more of aromatic rings in repeating units of the above-mentioned polyaniline are bonded at the meta-positions.
The aromatic rings in the repeating units of the above-mentioned polyaniline may have at least one substituent.
The above-mentioned proton conducting polymer is preferably obtained by doping the polyaniline with a strong acid or an acid compound.
The above-mentioned strong acid or acid compound is preferably an inorganic acid or an organic phosphoric acid compound.
The above-mentioned inorganic acid is preferably phosphoric acid and/or sulfuric acid.
Further, the invention provides a method for producing a proton conducting polymer, which comprises dissolving a polyaniline and an organic phosphoric acid compound in a common solvent, and casting the resulting solution.
Still further, the invention provides a solid polymer electrolyte comprising the above-mentioned proton conducting polymer.
Yet still further, the invention provides an electrode comprising the above-mentioned proton conducting polymer and fine catalyst particles carried on porous particles.